Billions of pounds of polyethelene and polypropylene are processed into pellet form annually. The pelletizing operation is accomplished with three basic components: an extruder, a die body with a wear resistant surface, and knives. The extruder's function is to melt the various plastics and homogeneously mix additives into this melt which enhance the properties of the plastic. This melt is then forced through the die under very high pressure. Pre-drilled holes in the die create strands of the melt which solidify as they contact the water environment which surrounds the die body. Knives pass in intimate contact with the face of the die and cut the strands of plastic into pellet form which then float to the surface of the water and are collected.
Die face materials, in combination with knife materials, play a critical role in determining the productivity and profitability of this extrusion process. Die face materials must be hard, wear resistant, and easily repaired or replaced in order to allow for multiple use of the intricate and expensive die body. Die face materials are also required to be corrosion resistant and must possess a combination of temper resistance, thermal shock resistance, and a low thermal conductivity. The two materials which meet that criteria, and are the most commonly used in the industry, are titanium carbide based metal matrix composites--generally titanium carbide (such composites are for convenience denoted Fe/TlC), and Tungsten Carbide/Cobalt (WC-Co) alloys.
Until recently, WC-Co alloys offered one very distinct advantage over Fe/TiC. WC-Co can be silver soldered or brazed, in air, to the die body with a resulting bond strength sufficient to withstand the pressures applied in extrusion operations. This is a relatively simple and inexpensive process. On the other hand, Fe/TiC die face material usage has been limited due to the difficulty of bonding it to the die body. Fe/TiC cannot be silver soldered or brazed directly to the die by conventional methods. It instead requires a sophisticated vacuum brazing process performed at very high temperatures. Even with the vacuum brazing process, the success rate was unpredictable and all too often the bonding was non-uniform or incomplete. In some cases, where an incomplete bond allowed plastic to flow into the gap between the die face and the die body, catastrophic failure occurred. When a die face fails, the extruder must be shut down and overhauled. The downtime cost for one of these units is several thousand dollars per hour.
It is a serious disadvantage of the tungsten carbide grains in the WC-Co alloys that they are generally angular with sharp edges, whereas titanium carbides in Fe/TiC alloys are smooth and rounded. These rounded carbides provide a "slippery" surface which greatly reduces metal-to-metal contact in rubbing wear applications. In contrast the angularity and sharp edges of the tungsten carbide grains tend to tear and cut mating materials at the microscopic level, for example, the faces of the knives. The rounded grains of Fe/TiC are less wearing. The mass change of mating materials run against the Fe/TiC is often nearly 40% less than the same material run against a WC-Co surface. In addition, the average coefficient of friction for an Fe/TiC surface is found to be approximately 19% lower than that of the WC-Co surface.
Both results illustrate the superiority of a Fe/TiC surface compared to a WC-Co surface for a die face material, especially, but not exclusively, for pelletizers. The low coefficient of friction means less force is required to rotate the knives against the die face, which in turn means that less heat is generated at the surface of the die. The reduction in frictional force and heat generation will reduce energy consumption. The low mass change of the mating material translates into longer knife life and reduced downtime. Finally, Fe/TiC alloys in general are almost 21/2 times less dense than WC-Co alloys. Heat transfer ratio is proportional to the density of a material. It follows that an additional reduction in energy consumption will be realized because the Fe/TiC will transfer less heat to the water which surrounds the face.
It is the object of this invention to provide an improved die face, reliably overlaid by Fe/TiC, and a method to make it.